Many production processes require mixing of liquids in an ultraclean operation. Such production processes may include the mixing of products such as pharmaceuticals, foods and chemicals. Certain of these may require aseptic processing. The term ultraclean as used herein refers in general to particularly stringent requirements for the levels of contamination which are acceptable in such processes.
Contamination in mixing processes may come from a number of sources. Among these are the mixing equipment itself and the cleaning processes which are invariably required during the use of such equipment.
One source of contamination comes from seals which may be required to seal a piece of equipment which must penetrate into the mixing vessel. Seals may be required, for example, around a rotary drive shaft to drive a mixer in the vessel. For this and other reasons, elimination of such seals is highly desirable. A mixer disclosed in U.S. Pat. No. 4,993,841 (Lofgren et al.) is an example of a mixer which eliminates the seal through the use of magnetic coupling of the rotary power in the mixer. Other mixer systems which use magnetic coupling to eliminate such seals are disclosed in the following U.S. Pat. No. 4,209,259 (Rains et al.); U.S. Pat. No. 5,79,359 (Gambrill et al.); U.S. Pat. No. 5,758,965 (Gambrill etal.); U.S. Pat. No. 6,568,844 (Arthun et al.); and U.S. Pat. No. 6,854,877 (Hoobyar et al.).
Another source of contamination is the relative movement of bearing surfaces against one another. This is particularly true when the bearing surfaces are not surrounded by liquid to provide lubrication to the bearing surfaces. When a mixing vessel is nearly empty of the product being mixed (mixing typically takes place while the product is being transferred from the mixing vessel into other containers), the bearing surfaces within the mixer run “dry.” During this period of operation, wear particles are more easily generated and then find their way into the product, either in the current batch of product or in a subsequent batch.
The cleaning of the mixing vessel and other equipment is also a source of contamination. Product which may spoil can become trapped in areas which are hard to reach during the cleaning process. Thus, it is desirable to be able to reach every area within a piece of equipment with the cleaning fluid being used. Sterilization as part of the cleaning process also requires the equipment to withstand high temperatures.
Current magnetically-coupled mixers, while eliminating the penetration of the mixing vessel from a drive shaft, still produce contamination caused by dry bearing contact, and because of such bearing contact, also contain areas within the mixer which are difficult to clean.
It is also desirable is such mixers to be able to transfer a high level of rotational mechanical energy into the fluid being mixed. Under such conditions, the fluid dynamic forces on the mixing element can be both large and rapidly changing due to such variables as high liquid viscosity, high mixing rates, and turbulence. Under such conditions, it is necessary that the mixing element be well supported by the bearings within the mixer and that the strength of the magnetic coupling be high enough to transmit the required driving forces, particularly during periods of high acceleration.